In cell-to cell handover in a radio communication system, a mobile station receives a list of information on peripheral base stations (hereinafter called “peripheral base station list”) from a base station in communication therewith. Then, the mobile station decides a cell (base station) to which the mobile station is to be handed over, by performing a cell search using the peripheral base station, list. Specifically, the mobile station searches out a base a station providing the largest received power of a base station reference signal (for example, a pilot signal) among base stations included in the peripheral base station list.
As a result the base station providing the largest received power of the base station reference signal is selected as a next connection target, that is, a handover destination base station. Meanwhile, when there is no base station providing received power of the base station reference signal which exceeds a threshold, the mobile station is subject to out-of-service processing.
The mobile station performs a cell search on all the base stations included in the peripheral base station list. Thus, time required for a cell search increases in proportion to the number or base stations included in the peripheral base station list.
Accordingly, the number of base stations included in the peripheral base station list is set to be small to some extent. For example, the peripheral base station list held by a base station includes information on other base stations within an area where base station reference signals transmitted from the base station can be received at a predetermined received power (for example, around −20 dB).
When the mobile station movements at a high speed, a movement range of the mobile station per unit time is large. Hence, it is desirable to increase the number of base stations included in the peripheral base station list. However, the increase in the number of base stations included in the peripheral base station list increases time required for a cell search. This time increase may cause a failure to keep up with the movement speed, or increase power consumption of the mobile station.
Against such a problem, an approach is proposed in which a cell size of a handover destination base station (an area covered by a base station) is selected depending on the movement speed of the mobile station (see Patent Document 1). Specifically, a mobile station moving at a high speed is handed, over to a base station with a large cell size, while a mobile station moving at a low speed is handed over to a base station with a small cell size.
Meanwhile, a multiband radio communication system is known as a radio communication system using multiple frequency bands. In multiband radio communications, frequency bands are spaced from one another so as to have different propagation and diffraction characteristics.
In such multiband radio communication system, an approach is proposed in which a frequency may band to be used by a handover destination base station is selected depending on the movement speed of the mobile station (see Patent Document 2). To be more specific, a relatively preferable communication quality can be maintained with a low frequency band even when moving at a high speed. Thus, a high radio-frequency band is allocated to a mobile station moving at a high speed, while a low radio-frequency band is allocated to a mobile station moving at a low speed.
Patent Document 1: Japanese Patent No. 2669288
Patent Document 2: JP-A 2003-70055